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A general guide for using the Forum with some rules and tips
Please help me identify what I have
  
  Index -> FOR BEGINNERS: What is it? Where is it from?
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Jadeybird




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PostPosted: Dec 20, 2018 14:37    Post subject: Please help me identify what I have  

It's a long story, so I will keep it as short as I can. My girlfriends roommate passed away with no next of Kin. She asked me to help clean out his room and pack up his stuff to donate to good will. Underneath his mattress we found a zip lock bag full of these stones. Written on the front of the bag in black marker was "$5K". The man owned a gem and bead store before he fell ill. We think these may be from his store. All in total the bag weighs 2013 grams. These are really heavy stones. I was able to break one up with a hammer but it wasn't easy. I brought them to the Gem and Minerals Society in Balboa Park and the first answer I was given was black spinel. However when tested for refraction the results were not matching up with spinel. So I still had no answer. Anyone have an opinion on this?


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Bob Harman




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PostPosted: Dec 20, 2018 15:12    Post subject: Re: Please help me identify what I have  

Based primarily on the last 2 photos, many of your examples look like magnetite octahedrons. I can't rule out other black stones or even some black slag admixed, but most of the stuff appears to be the black magnetite octahedrons.

Magnetite is magnetic. Put a small iron paperclip or small iron nail etc (it must be iron!) up to the rocks and see if the clip or nail sticks. If so, that helps with the ID.
BOB
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kushmeja




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PostPosted: Dec 20, 2018 15:22    Post subject: Re: Please help me identify what I have  

I was also thinking Magnetite, or possibly Franklinite
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Jadeybird




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PostPosted: Dec 20, 2018 15:48    Post subject: Please help me identify what I have  

It's a long story, so I will keep it as short as I can. My girlfriends roommate passed away with no next of Kin. She asked me to help clean out his room and pack up his stuff to donate to good will. Underneath his mattress we found a zip lock bag full of these stones. Written on the front of the bag in black marker was "$5K". The man owned a gem and bead store before he fell ill. We think these may be from his store. All in total the bag weighs 2013 grams. These are really heavy stones. I was able to break one up with a hammer but it wasn't easy. I brought them to the Gem and Minerals Society in Balboa Park and the first answer I was given was black spinel. However when tested for refraction the results were not matching up with spinel. So I still had no answer. Anyone have an opinion on this?


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Jadeybird




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PostPosted: Dec 20, 2018 15:55    Post subject: Re: Please help me identify what I have  

Not magnetic at all
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Pete Richards
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PostPosted: Dec 20, 2018 16:03    Post subject: Re: Please help me identify what I have  

Try it with a magnet. Not all magnetite is itself magnetized, but it will be attracted to a magnet. Franklinite should be slightly attracted.

Still, I doubt if it is magnetite (or franlklinite).

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Jadeybird




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PostPosted: Dec 20, 2018 16:11    Post subject: Re: Please help me identify what I have  

I did, I used an earth magnet. And they tested it at the museum as well
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Kevin Conroy




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PostPosted: Dec 20, 2018 16:29    Post subject: Re: Please help me identify what I have  

Do you still have the pieces of the one that you broke with a hammer? If so please post a picture of them.

Can you see color when shining a bright light through them? I ask because one looks a little reddish in the first photo. Cuprite maybe?

Do you know where the store was?
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Jadeybird




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PostPosted: Dec 20, 2018 16:33    Post subject: Re: Please help me identify what I have  

Its the one I posted close up. There's no transparency at all. Some of the stones have white dots on the outside and some do not. The store is closed now but was in san diego
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texasdigger




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PostPosted: Dec 20, 2018 16:39    Post subject: Re: Please help me identify what I have  

If you have a digital scale you can try a specific gravity test. At the very least it will rule out a lot of minerals.

You need to weigh the stone two different ways.
First, the actual weight.
Second, fill a small cup with water put on scale and zero it out. Use floss or string to suspend the rock in the water without touching sides or bottom. This will be the displaced water weight. Divide actual weight by displaced water weight to get a specific gravity.
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Kevin Conroy




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PostPosted: Dec 20, 2018 17:12    Post subject: Re: Please help me identify what I have  

It will help if we know the hardness. For example, will a sharp edge of it scratch glass? For more info google "mineral hardness test"
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Pete Modreski
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PostPosted: Dec 20, 2018 18:32    Post subject: Re: Please help me identify what I have  

Interesting question! When I first saw your picture(s) of the specimens, my first thought was, indeed, black spinel. You can see crystal faces, that look like octahedra, on many of the crystals. I did not think magnetite--it does not look "submetallic" enough, so I'm not surprised that you say they are not magnetic.

Now, gem spinels (which of course would be what gemologists normally measure the r.i. of), near MgAl2O4 in composition, are low in iron content and transparent, and might be shades of pink, red, violet, blue, or colorless, and would have a refractive index near 1.72. But, iron-rich spinel would appear black, opaque or nearly so, and would have a much higher refractive index; for example, hercynite, ideally FeAl2O4, is listed as having an r.i. of 1.835; the the spinel compositions could be anywhere between these.

I have seen black spinel crystals that erode out of alkalic basaltic lavas, and typically have a crude octahedral shape, much like your crystals; and these would be quite iron-rich. And they would have no cleavage, and break to show a glassy-looking interior. This would be my "first guess" as to what your crystals are; there are, of course, many other black, heavy minerals with octahedral crystal shape.

As has been noted, a specific gravity test (even if fairly crude) should help pin down what these may be. Sp. gr. of "ordinary" spinel is 3.55; hercynite, 4.40 (magnetite is 5.2).

Here is one article talking about black spinel crystals in alkali basalt (from Thailand):
https://rockingwiththerocks.com/khao_ploi_waen.html It includes a picture or two (not very close up) of rounded black crystals in the basalt.

Best of luck! Let us know if you find out any more.
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Jadeybird




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PostPosted: Dec 20, 2018 19:40    Post subject: Re: Please help me identify what I have  

Wow thank you for all of the help and expert advice. I appreciate your time spent on this. Now, I did the test to get the specific gravity weight and came out with 3.9. What does that tell us?


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Jadeybird




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PostPosted: Dec 20, 2018 19:45    Post subject: Re: Please help me identify what I have  

And also it leaves no streak at all. Quite difficult to scrape it
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Kevin Conroy




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PostPosted: Dec 20, 2018 20:56    Post subject: Re: Please help me identify what I have  

I think that Pete nailed it. The specific gravity range. crystal shape and hardness all point toward spinel (or a group member).
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Mathias




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PostPosted: Dec 21, 2018 01:09    Post subject: Re: Please help me identify what I have  

Sphalerite
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Stefan Koch




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PostPosted: Dec 21, 2018 03:38    Post subject: Re: Please help me identify what I have  

Hi

Pretty sure this is spinel!

Cheers
Stefan
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Mathias




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PostPosted: Dec 21, 2018 03:45    Post subject: Re: Please help me identify what I have  

Could also be cassiterite.
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Tobi




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PostPosted: Dec 21, 2018 04:15    Post subject: Re: Please help me identify what I have  

Why does this discussion exist twice? It is already discussed here:
https://www.mineral-forum.com/message-board/viewtopic.php?p=65237#65237
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Jordi Fabre
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PostPosted: Dec 23, 2018 15:00    Post subject: Re: Please help me identify what I have  

Tobi wrote:
Why does this discussion exist twice? It is already discussed here:
https://www.mineral-forum.com/message-board/viewtopic.php?p=65237#65237

I fused both threads so now we have just a proper one.

I would also say Spinel (variety pleonaste)
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